1. Field of the Invention
The present invention relates to a method for forming liquid crystal display (LCD), and more particularly to a method for overcoming shadowing caused by a glue seal which is formed between first and second LCD panels during the formation of the LCD.
2. Description of the Related Art
Novel manufacturing methods are actively being explored to increase the throughput of LCD panels while decreasing the cost of liquid crystal display panels. Both cost and throughput are expected to be advantageously affected by using the so called xe2x80x9cone drop fillxe2x80x9d (ODF) method, a manufacturing process first described in U.S. Pat. No. 5,263,888, incorporated herein by reference.
Here, the liquid crystal is contained between two substrates that comprise the panel prior to sealing of the substrates. A peripheral fillet of uncured glue seal temporarily keeps the liquid crystal in place between the two substrates forming the liquid crystal display prior to polymerizing or curing of the sealant.
Since, in the ODF process, sealing of the edges of the two affixed substrates comprising the panel occurs with the liquid crystal already in place, the conventional oven thermal cure process for polymerizing the glue seal is generally not applicable for preferred panel sealants. Instead, the glue sealant is one that cures photolytically, customarily stimulated by incident ultraviolet radiation which in turn activates photoinitiators to chemically react to polymerize or cure the glue sealant. In this manner, overheating of the liquid crystal material in contact with the glue seal is prevented which would occur if conventional oven curing were used with the ODF process.
However, currently, a pressing problem is to overcome the shadowing that occurs when liquid crystal panels are sealed using light (typically ultraviolet (UV)) to produce photoinitiators that lead to curing or polymerization of the glue seal.
While a number of methods have been suggested to overcome this problem, no known method is inexpensive, easy to apply, or completely effective.
In view of the foregoing and other problems, drawbacks, and disadvantages of the conventional methods and structures, an object of the present invention is to provide a method and structure in which the problem of shadowing is overcome.
Another object is to provide a method and structure for directing radiation incident on an otherwise transparent substrate to regions underneath the areas of opacity.
In a first aspect of the present invention, a system (and method) includes a first substrate having both a transparent region and alternating opaque and transparent regions with respect to incident electromagnetic radiation, a radiation diffuser at least partially transparent to the incident electromagnetic radiation, a coupler for attaching the radiation diffuser to the first substrate to form a diffuser-substrate interface, a polymer used for affixing the first substrate to a second substrate, the polymer positioned between the first substrate and the second substrate along at least a peripheral region common to both the first and the second substrates, and a source of electromagnetic radiation incident onto the diffuser attached to the substrate for polymerizing the polymer.
With the unique and unobvious aspects of the present invention, a method is provided which is extremely inexpensive and simple to apply.
That is, the present invention provides a structural mechanism for directing radiation incident on an otherwise transparent substrate to regions underneath the areas of opacity. The opacity prevents radiation from being directed into and through the substrate by radiation incident in a direction normal to the substrate.
To solve the above-mentioned problems of the conventional methods, two inventive approaches are described.
A first approach uses diffusing elements that scatter radiation directed at normal incidence. The diffuse scattering elements are attached by a pressure-sensitive adhesive, and can be readily removed after irradiation.
A second approach uses a novel set of scanners configured in a manner that directs, preferably p-polarized radiation, at a relatively steep angle with respect to the surface normal (e.g., a relatively shallow angle with respect to the substrate surface) to the substrate to irradiate underneath the otherwise opaque region.
It is noted that, while several embodiments and applications are possible, the invention is described in terms of optical radiation required to reach areas underneath metallized or opaque regions on an otherwise transparent substrate. As described above, this problem arises especially, though not exclusively, in the manufacture of liquid crystal flat panel displays. Here, UV light is incident on the periphery of the panel to create photoinitiators to promote the curing of the polymeric glue seal that binds the two substrates comprising the flat panel display. However, metallization along certain regions of the substrate onto which the radiation is incident prevents the UV from reaching underneath the opaque metallization, thereby leaving unpolymerized sealant that can lead to contamination of the liquid crystal.
Hence, the present invention directs electromagnetic radiation for polymerizing or curing a sealant to affix two substrates to one another. For the preferred embodiment, ultraviolet radiation is directed at a first substrate having both transparent and alternating opaque and transparent regions with respect to the incident electromagnetic radiation.
To utilize radiation that is directed at or near normal incidence to the first substrate and yet have at least a component of radiation that penetrates beneath the opaque region of the first substrate, a radiation diffusing element, at least partially transparent to the electromagnetic incident radiation is attached to the first substrate in the regions of alternating opaque and transparent regions by a pressure sensitive adhesive with the pressure sensitive adhesive on one side of the diffusing element, the adhesive positioned intermediate to the diffuser-substrate interface.
Since the adhesive will be closely index-matched to the indices of refraction of both the diffuser and the substrate, the diffused radiation resulting from incident radiation directed at the substrate with the attached diffusing element will not undergo any substantial change of direction at the diffuser-adhesive or adhesive-substrate interfaces. Thus, the diffusely spread incident beam will remain diffusive as it passes into the substrate so that some radiation will penetrate underneath the opaque or shadowed region. Total internal reflection at the diffuser-substrate interface will be circumvented by the index matching at the diffuser-adhesive-substrate interfaces.
Preferably, a polymer sealant is used for affixing the first substrate to a second substrate along a peripheral region common to both substrates, the polymer reacting to an incident source of radiation to cause polymerization or curing of the polymer. After the polymerization step the diffusing element is removed from the substrate for possible re-use.
The diffusing element attached to the first substrate may include a tape with a matte finish to diffusely scatter incident light.
The diffusing element attached to the first substrate may include a grating tape.
Alternatively, a sheet (e.g., a polymer sheet) with an imprinted hologram can be affixed to the substrate to act as a diffusing element. The hologram can be designed to cause incident radiation, particularly normally incident collimated radiation to diffuse or spread into a large cone angle essentially acting as a negative lens, thereby directing at least a portion of the incident radiation underneath the opaque or shadowed region to cure the shadowed polymer between the first and second substrates. The polymer sheet with the imprinted hologram also may have a pressure-sensitive adhesive which allows the hologram to be attached to the first substrate and removed after the polymerization has been completed.
Alternatively, the invention includes methods that direct radiation at steep angles with respect to the normal of the first substrate onto a transparent or partly transparent substrate with or without the use of adhesive means for attaching an intermediate refracting element, the substrate including alternating opaque and transparent regions. As before, the substrate may be part of a panel comprising two substrates with a polymer glue seal positioned between the two substrates along one or more peripheral edges.
In one preferred embodiment, the bending of normally incident electromagnetic radiation is achieved through a thin transparent polymer sheet having periodic, closely spaced prismatic linear structures on the side of the transparent sheet facing and in contact with the first substrate. The prismatic structures are shaped to permit radiation directed at or near normal incidence from the smooth or non-prismatic side of the transparent sheet to be internally reflected from one face of the prism, then refracted or bent away from the incident normal to the prismatic structure""s surface into air.
The resulting radiation enters the substrate at a near glancing angle with respect to the substrate""s surface to penetrate beneath regions of opacity in order to polymerize the polymer glue seal along the periphery of the first and second substrates in both the alternating clear and opaque regions.
The preferred incident radiation for this embodiment is p-polarized as it is less reflected than s-polarized light at large incident angles with respect to the substrate incident normal. Thus, relatively more of the p-polarized light will enter the substrate for the intended purpose of polymerizing the glue seal. A commercially available material with this type of prismatic structure is manufactured by 3M(trademark) and marketed under the tradename TRAF II(trademark) where TRAF is an acronym for Transmissive Right Angle Film.
Another preferred embodiment that refracts radiation at large angles with respect to the incident normal or near glancing angles with respect to the substrate again makes use preferably of p-polarized radiation. Here, no intermediate refracting element is between the radiation source and the first substrate.
To obtain this embodiment, a flexible optical fiber is attached to a computer controlled robotic arm where one end of the fiber receives electromagnetic radiation from a source of radiation (e.g., a laser). The opposite end of the fiber from which the radiation exits is directed onto a mirror or reflecting surface also attached to the robotic arm. The reflector is tilted preferably at an angle such that the reflected radiation is directed at the substrate containing the opaque and clear regions at an angle of incidence with respect to the normal to the substrate in the range of about 40 degrees to about 90 degrees.
Again, for p-polarized radiation only a small fraction of the radiation on the order of 10-20% will be reflected at these incident angles from the substrate so that a substantial amount of radiation will enter the substrate and penetrate under shadowed or opaque regions. The robotic arm is scanned along the panel periphery so that the entire glue seal receives radiation to polymerize the glue seal affixing the two substrates to one another. In all instances, the present invention directs radiation into regions that are not transmissive to normal incidence radiation (e.g., where radiation is required in regions generally difficult to access by standard mechanism).